Three dimensional (3D) imaging time-of-flight (TOF) cameras are an active-type system. In general, systems are based on the phase-measurement technique of emitted intensity-modulated light, which is reflected by the scene. The reflected light is imaged onto a sensor. The photo-generated electrons are demodulated in the sensor, and based on the phase information, the distance for each pixel is deduced.
A major problem of a TOF system is that the sensor has to handle high dynamic ranges. The modulated signal received by the camera drops with the square of the distance. Furthermore, the reflectivity of the targets might vary to a large degree.
FIG. 1 illustrates a scene with a high dynamic range. The image detected by TOF camera 100 contains a bright object 10 at 30 centimeters (cm) with a reflectivity of 100% and a dark object 12 at 300 cm with a reflectivity of 10%. Therefore, the dynamic range, for objects 10, 12 at the same distance, becomes:
  DR  =                              300          2                *        100                              30          2                *        10              =                  1        ′            ⁢      000      
Due to this high requirement on dynamic range, stray light originating from the strong signal adding to the weak signal is a dominant problem for numerous applications of the TOF technology.
Stray light in TOF systems can also come from the camera itself.
FIG. 2 illustrates stray light that is generated by the non-ideal optical path 14 in the camera system 100. A non-ideal path 14 is sketched that originates from reflections between the lens (objective) system 110 and the sensor or imager chip 200. However, stray light might also have been generated by the lenses inside the objective or by an optical filter such as a bandpass filter added in the optical path.
Again, the stray light problem is exacerbated by the presence of relatively bright (reflective) objects 10 and dark (absorbing) objects 12 within the same scene. Light 16 from the bright object 10 contributes to the response detected by the pixels that receive the light 18 from the dark object 12 as illustrated by the internal reflections that give rise to the non-ideal path 14.
FIG. 3 illustrates the impact of stray light for the case of a phase-measuring 3D TOF system.
Signal A from object A (10) on pixel A: strong signal=large amplitude
Signal B from object B (12) on pixel B: weak signal=small amplitude
S=Signal due to stray light from object A on pixel B
B′=Resulting (measured) signal on pixel B